Nitric oxide (NO) is a diffusible, reactive molecule that has many overlapping biological functions, including control of vascular tone and blood pressure, protection against pathogens and cancer, hormone regulation, nerve cell transmission, and angiogenesis. Nitric oxide synthase (NOS) proteins are heme-based monooxygenase enzymes that convert L-arginine to L-citrulline and nitric oxide (NO) by a two-step electron transfer process. Mammalian NOS enzymes are homodimers that contain an N-terminal oxidase domain (NOSox) and C-terminal reductase domain called NOSred. Crosstalk between the two domains is regulated by a calmodulin (CaM)-binding interface. NOSox binds the L-arginine substrate, heme, and the redox-active cofactor 6R-tetrahydrobiopterin (H4B), all of which are required for an active enzyme. NOSred has binding sites for flavin cofactors as well as NADPH, and acts as a source of reducing equivalents for oxygen binding and activation at the heme in NOSox. Controlling the communication between redox-active cofactors in the NOSox and NOSred domains regulates at least two mammalian NOS isozymes, though a structure of the two domains in complex has not yet been achieved. Bacterial NOS enzymes share many similarities to their mammalian counterparts, and because of their stripped-down domain structure and ease of purification, bacterial NOS proteins serve as useful models for investigating the mechanism of NO synthesis. The goal of this proposal is to provide a better understanding about the relationship between NOS structural arrangement, electron transfer and the mechanism of NO production by NOS enzymes. In aim 1, we will study a novel NOS enzyme from S. pcc7335 (spNOS), characterizing its steady state activity and yield of NO synthesis, the reaction kinetics of its NOSox domain, as well as the affinity and specificity of pterin substrates for its redox active site. In aim 2, we will obtain crystal structures of two bacterial NOS enzymes, spNOS and a NOS enzyme from S. cellulosum (scNOS), which contain a fused reductase domain never observed before in bacterial systems. Finally, Aim 3 will target specific redox intermediates in the NOS electron transfer mechanism for structural characterization. Specifically, we will determine detailed structures of two heme-oxy states occurring in G. stearothermophilus NOS (gsNOS).